1. Field of the Invention
The present invention relates to a sensor device and to a method for fabricating the sensor device and more specifically to a sensor device in which insulating layers are provided between a frame and a beam, and between a dead-weight and the beam, and to a method for fabricating the sensor device.
2. Description of Related Art
MEMS (Micro Electro Mechanical Systems) are functional parts in which mechanical elements are combined with electrical elements. There is a piezoresistance-type acceleration sensor as one example of the MEMS (see Japanese Patent Application Laid-Open No. 2005-49130 for example).
FIG. 9 is substantially a section view of a piezoresistance-type acceleration sensor. When force caused by acceleration is applied to a dead-weight portion 501 of the sensor structure 1000, a beam portion 507 connected with supporting portions 505 and a mass portion 506 deflects, and a resistance value of a piezoresistance element 508 assembled in the beam portion 507 is changed. The changes of current or voltage caused by the changes of the resistance value are taken to the outside from an electric wire (not shown) connected to the piezoresistance via a metal pad 509 formed on an upper face of the supporting portion 505. It becomes possible to detect the acceleration by detecting this change.
Furthermore, the supporting portion 505 is provided at the frame portion 502 via an insulating layer 503 made of an oxide film and the mass portion 506 is provided at the dead-weight portion 501 via an insulating layer 504 made of an oxide film.
However, although there is a step of etching an under part of the beam portion 507 to form the beam portion 507 in fabricating the piezoresistance acceleration sensor having the structure described above, there has been no thought of etching edges of the bottom faces of the supporting portion 505 and the mass portion 506 to relax concentration of stress applied to the edges of the beam portion 507. The edges of the bottom faces of the supporting portion 505 and the mass portion 506 are slightly etched accidentally in finally made products. A space 510 between the side face of the supporting portion 505 and the side face of the insulating layer 503, i.e., the part slightly etched, is very narrow. Therefore, it is almost the same thing to support the beam portion 507 by a side of the edge of the upper face of the insulating layer 503 and a side of the edge of the upper face of the insulating layer 504. Accordingly, there has been a problem that when a sudden impact is applied, the stress concentrates on a boundary part between the supporting portion 505 and the beam portion 507 and on a boundary part between the mass portion 506 and the beam portion 507, easily damaging the beam portion 507 by the impact.
In order to avoid such problem, the improvement of the impact resistance has been made by adjusting the width, thickness and others of the beam portion in the past.
However, it has been difficult to achieve the both micronization and improvement of the impact resistance because the impact resistance deteriorates if a width of the frame is reduced along the reduction of the acceleration sensor, a width of the beam is reduced along that or a thickness of the beam is reduced for the purpose of enhancing its sensitivity.